Tramp oil is the biggest reason for stinky coolant. It's like covering up a dog turd with a plastic bag. That's why bubblers work, it breaks the surface tension. The other main reason I see coolant go bad is from how it's mixed, or not for that matter. Adding straight water is one of the biggest issues, you always want some mix, for the biocide in any half modern coolant. Another issue is how the coolant is mixed, always add concentrate to water, not water to the concentrate. Adding water to concentrate can cook off the biocide,, and can cause that jelly like separation.
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